That clause is exactly a forswearing of power not an exercise of power

A federal law is not an exercise of power? It may reject applying one states law to another but that in and of itself is exercising federal powers.

Now, Why did that have to get passed in the first place? Ostensibly, under this whole 50 different nation states and a small constitutional federal govt, that wasnt needed in the first place. Dont agree? Why do we have state constitutions and govts then?

"Under the law, no U.S. state or political subdivision is required to recognize a same-sex marriage from another state." (emphasis added)

That clause is exactly a forswearing of power not an exercise of power.

A federal law is not an exercise of power?

Not when its core is a nonrequirement.

It may reject applying one states law to another but that in and of itself is exercising federal powers.

Preventing one state from violating the sovereignty of another state would be a valid exercise of federal power.

Now, Why did that have to get passed in the first place? Ostensibly, under this whole 50 different nation states and a small constitutional federal govt, that wasnt needed in the first place.

The concern was that otherwise the Full Faith and Credit Clause would be interpreted to mean that all states had to recognize gay 'marriages' performed in states that allowed it. Did you really not know that?